1. Field of the Invention
The present invention relates to a multi-core amplification optical fiber.
2. Description of the Related Art
In order to cope with a rapid increase in a data transmission volume in recent years, a multi-core optical transmission line that uses a so-called multi-core optical fiber in which a plurality of core portions is arranged in the cladding of one optical fiber has been actively developed. Accordingly, development of a rare-earth-doped optical fiber amplifier for amplifying signal lights having propagated through the multi-core optical transmission line is requested.
For example, Japanese Laid-open Patent Publication No. 2005-19539 discloses a multi-core amplification optical fiber for an optical fiber laser, in which a plurality of rare-earth-doped core portions is disposed in the cladding. Moreover, Japanese Laid-open Patent Publication No. 10-125988 discloses a multi-core optical fiber amplifier for amplifying signal lights in a lump, the signal lights having propagated through a multi-core optical transmission line.
Meanwhile, some conventional rare-earth-doped amplification optical fibers in which one core portion is disposed near a central axis of an optical fiber employ a double-cladding structure. In the double-cladding structure, it is known that, when an inner cladding has a circular cross-sectional shape, a certain component (skew component) of a pumping light may not reach the core portion and does not contribute to pumping, and thus, pumping efficiency is not satisfactory. Therefore, a method of forming an inner cladding so as to have a star-shaped, polygonal, or D-shaped cross-section is used in order to disturb the skew component so as to be efficiently absorbed in the core portion (see Japanese Laid-open Patent Publication No. 2003-226540).